Acetal resins have been employed as substitutes for metals for inner machinery parts, such as gears, bearings, sleeves, shafts, etc., due to their high mechanical strength, hardness, friction and wear resistance, heat resistance, chemical resistance, fatigue properties and the like, and application of the acetal resins to exterior parts of automobiles or electric devices have recently been increased.
Therefore, it has been required for the acetal resins to have further improved weather resistance or surface gloss enough to be used as exterior parts. Further, in order to meet requirements for performances such as improvement in heat resistance or dimensional stability and decreased shrinkage in molding, it has been demanded that the acetal resins be used as composite materials in combination with various fillers.
According to findings so far developed, the acetal resins should be molded by injection using a mold set at a relatively high temperature in order to prevent formation of flow marks on the molded products and to increase surface gloss of the molded products. Molding in a high temperature is, however, disadvantageous in that the molding cycle is long; the molded products have increased shrinkage and increased warpage, etc.
It is known that the acetal resins can be used as composite materials by mixing with organic or inorganic fillers such as glass fibers, glass powders, carbon fibers, potassium titanate fibers, metal carbonates, e.g., calcium carbonate, silica, metal sulfides, polytetrafluoroethylene, carbon black and the like, as disclosed, for example, in U.S. Pat. No. 3,775,363 and Japanese Patent Publication Nos. 28191/69, 25181/70, 25184/70 and 7615/64. However, mere mixing of acetal resins with these fillers often causes problems such as decreased heat stability and decreased mechanical strength.
In order to overcome the above-described problems, an improved process has been proposed in which these organic or inorganic fillers are coated with a phenoxy resin, polyamide, polyurea, polystyrene, polyvinylpyrrolidone, polyurethane, etc., and then mixed with acetal resins. When glass fibers or glass powders are used as fillers, a process of using isocyanates, polycarbodiimides, alkoxymethylmelamines, etc., has also been known, as disclosed, for example, in U.S. Pat. Nos. 3,455,867, 3,647,743 and 4,111,887, GB Pat. Nos. 1,297,458 and 1,331,829, Japanese Patent Publication Nos. 31744/71, 18741/80, 9393/82 and 18383/83, and Japanese Patent Publication (Unexamined) No. 157645/80. However, these known processes are disadvantageous in that the desired effects are not satisfactorily achieved or the composite materials greatly contaminate a mold during injection molding.
Further, although the isocyanates, polycarbodiimides and alkoxymethylmelamines are effective in improving adhesiveness between silane-treated glass fibers or glass powders and acetal resins, no effect is observed in the improvement of adhesiveness between acetal resin and those fillers on which the silane treatment is not effective, such as carbon fibers, carbon black, metal carbonates, metals, tetrafluoroethylene resin and the like.
Furthermore, a large amount of carbon black has conventionally been incorporated into the acetal resins in order to increase weather resistance, but such gives rise to significant reduction in heat resistance and mechanical strength, and cannot provide satisfactory molding materials. In addition, it is difficult to incorporate metal oxides, metal silicates, metal carbonates and the like to acetal resins for imparting a light screening property to the acetal resins.
Thus, the production of composite materials composed of acetal resins and fillers according to conventionally known processes have many problems.